FIG. 1 shows a conventional converter having three half-bridge branches B1, B2 and B3 which have in each case an upper bridge branch B1T, B2T, and B3T, respectively, a lower bridge branch B1B, B2B and B3B, respectively, and a phase output PH1, PH2 and PH3, respectively. The phase outputs PH1, PH2 and PH3 are connected to a common load M, for example a motor. With respect to the individual bridge branches B1, B2 and B3, the load M has in each case a load inductance L1, L2 and L3, respectively. Since the inductances involved are in the foreground for the following considerations, the representation of ohmic components of the load M and of the other components was omitted, but they are still unavoidably present.
Each of the three half bridges B1, B2 and B3 includes two controllable semiconductor switches Z1/Z2, Z3/Z4 and Z5/Z6, respectively, the load current paths of which are connected in series. A node common to the load current paths forms the phase output PH1, PH2, PH3. In the embodiment shown, the load inductances are interconnected in the shape of a star, i.e. each of the inductances L1-L3 is connected with one terminal to a common star point. As an alternative, the load inductances can also be interconnected triangularly. In this case, each inductance L1-L3 is connected between two of the phase outputs. A freewheeling diode D1, D2, D3, D4, D5 and D6, respectively, is connected in each case antiparallel to each of the semiconductor switches Z1, Z2, Z3, Z4, Z5 and Z6, respectively. To implement the converter, the half-bridges B1, B2 and B3 are in each case connected to a link circuit voltage V1. In addition, a link circuit capacitor C1, C2 and C3 is connected in parallel with each of the half-bridges B1, B2, B3.
To electrically interconnect the individual components within the individual bridge branches B1, B2, B3, conductor tracks, terminal plates and the like must be used, as a result of which stray inductances are unavoidably formed. Some of these stray inductances L4 to L9 are shown by way of example in FIG. 1. Since, due to these stray inductances L4 to L9, high induction voltage peaks occur within the individual bridge branches during the commutation, which, in particular, load the semiconductor switches Z1, Z2, Z3, Z4, Z5, Z6 and the free wheeling diodes D1, D2, D3, D4, D5, D6 and can thus lead to overvoltages and to oscillation phenomena in the entire system, it is attempted in conventional power semiconductor assemblies to keep the stray inductances L4 to L9 as low as possible within the individual bridge branches B1, B2, B3.
Further improvements can be achieved in that inductances L10, L11, L12, L13, L14, L15 of terminal and connecting lines, which extend essentially perpendicularly away from the assembly plane of the semiconductor switches Z1, Z2, Z3, Z4, Z5 and Z6 in a module containing the semiconductor switches, are kept as short as possible and run in parallel and symmetrically.
Despite these improvements, interfering induction voltage peaks can still occur. In converters for high-current applications, the semiconductor switches Z1-Z6 shown by electric switching symbols in FIG. 1 usually include in each case a number of switching elements connected in parallel and driven jointly. Between the individual parallel-connected switching elements, further parasitic inductances can be present in this arrangement. These further inductances can lead to an asymmetry with respect to the current loading of the individual parallel-connected switching elements.
In known converters, the individual bridge branches are usually arranged in a direction perpendicular to the main current direction of the bridge branches next to one another, which is explained with reference to FIG. 1.
The representation in FIG. 1 is selected in such a manner that it reproduces not only the circuit diagram of the arrangement but also the relative position of the controllable power switches Z1, Z2, Z3, Z4, Z5 and Z6 with respect to one another. Accordingly, each of the bridge branches B1, B2, B3 can be allocated a main current direction I1, I2 and I3, respectively, which is in each case indicated by an arrow and is approximately given by the direction of the connecting line between the two power switches Z1 and Z2, between the two power switches Z3 and Z4 and between the two power switches Z5 and Z6, respectively, of the respective bridge B1, B2 and B3. The individual half bridges B1, B2, B3 are arranged next to one another in a transverse direction Q which extends approximately perpendicular to the main current directions I1, I2, I3.
As a result, a considerable transient current flow in the transverse direction Q and therefore also induction voltage peaks which, in particular, also originate from the stray inductances L16, L17, L18 and L19 formed by the sections of the connecting lines extending in the transverse direction Q, are produced during commutation processes in the half bridges B1, B2, B3 due to the interconnection of the bridges B1, B2, B3 with one another in the connecting lines required for this purpose. In the text which follows, this is explained by way of example with reference to FIG. 2.
FIG. 2 shows the arrangement according to FIG. 1 in a particular operating state in which a current flow exists along a first path P1 represented by arrows and by an increased line thickness. With conducting power switches Z1 and Z4 and with nonconducting power switch Z2, the current flows from the link circuit voltage source V12 through the stray inductances L20, L16, L10 and L4, the power switch Z1, the inductances L1 and L2, the power switch Z4 and the stray inductances L7, L13 and L21 back to the link circuit voltage source V1.
During a subsequent commutation process, the power switch Z1 is placed into a non-conducting state, while the power switch F4 remains conducting. Immediately after switch-off, an induced current, the direction of which corresponds to the direction of the current through these inductances L1 and L2 before the switch-off of Z1, occurs in the inductances L1 and L2 due to the energy mainly stored in the inductances L1 and L2 of the external load M.
Since the power switch Z1 is switched off, the current caused by the load inductances L1, L2 flows along a second path P2, also represented by arrows and an increased line thickness, which is shown in FIG. 3. In this arrangement, the current flows, starting from the inductance L1, through the inductance L2, the switched-on power switch Z4, the stray inductances L7, L13, L18, L11 and L5 via the freewheeling diode D2 back to the inductance L1.
As can be seen by comparing FIGS. 2 and 3, the current through the stray inductances L4, L10, L16, L20 and L21 becomes zero due to the commutation process or at least drops significantly, while a current through the stray inductances L18, L11 and L5 is produced or increases significantly. These current changes in the inductances L4, L5, L10, L11, L16, L18, L20 and L21 occur very rapidly especially during “hard” switch-off of Z1. As such, high unwanted induction voltage peaks U4, U5, U10, U11, U16, U18, U20 and U21 occur which are dropped across the respective inductances L4, L5, L10, L11, L16, L18, L20 and L21. In the case of a similar commutation process, unwanted induction voltage peaks also occur in the other circuit parts in a corresponding manner.